Audio-noise responsive squelch circuit



July 24, 1956 D. J. BRAAK 2,756,328

AUDIO-NOISE RESPONSIVE SQUELCH CIRCUIT Filed May 1, 1952 DETECTOR RE AMPLIFIER and FREQUENCY CONVERTER Dirk Johan Brook AGENT 2,756,328 eva ac se SP N WE SQUEPCH. 39 3 Dirk Johan Braak, Hilversum, Netherlands, assignor to Hartford National Bank and Trust CompanyyI-Iartford, Conn., as trustee 1 The present invention concerns audio-noise responsive squelch circuits. More particularly, the invention relates'to circuit-arrangements for suppressing the noise'in Wireless receivers which occurs in the absence of a signal or during very weak signals.

Many'noise suppressing circuit-arrangements are already known. In most of these circuit-arrangements the main receiving channel, and more often the'low-frequencypart thereof, comprises a controlled discharge tube which, in the absence of a signal in the input circuit of -the receiver'or if the signal is unduly weak, is made inoperative due to a high negative voltage being applied toagrid of said tube. The occurrence of a signal voltage in the input circuit may cause the said negative voltage to be compensated by a positive voltage which may be produced, forexample, by an auxiliary amplifying tube, so that normal reception may take place.

" In receivers for frequency-modulated oscillations, a direct voltage derived from the noise voltages by amplification and detection may frequently be used to block the main channel in the absence of a signal. This utilizes the principle'that in the presence of a signal voltage the level of the noise voltages in the output circuit decreases as a result of the limiting elements becoming operative, 'sothat the blocking voltage decreases. In such receivers use may be made of a separate noise voltage amplifier, which also serves as a detector 'and whicli'p "ducesanode current in the absence of a signal. Itsfa od circuit "comprises a resistance which also forms part o f 'the grid' circuit of the channel blockingampliiier tube and which supplies a negative'gricl-bias. Thecon trol grid of the noise amplifying tube in the known circuits mayalsb be subject tothe action of a direct voltage dependent up'onthe signal voltage. "The use of a separate diode detector for detecting the amplifier noise voltages hasalsob'ee'n suggested.

"The e'ct of the present invention is to provide a circuit-a rigem'ent'for 'sup't'iressing noise in wirelessrec'e i'vers. Sinallvariationsin the rectified, amplified noise voltages provide 'a'variation in the 'biassing potential of the blocking tube o'r' tube's'which' issufiicient to bring about a complete change of said tube, from the blocked, 6Y non coiidncting'"condition, to the unblocked, or conducting condition! 1 in" awn-dance with the present invention, in a circuitarrangement for suppressingnoi's'e in a wireless receiver, noise voltages from a receiving channel are supplied to the control grid of a noise amplifying tube. In the absence of a signal, the rectified output voltage of the noise amplifying tube is active at the grid of each of one or more discharge tubes included in a further receiving channel to block said channel. The circuit-arrangement is so comprised that the level of the noise voltages decreases upon occurrence of a signal voltage and said further receiving channel is unblocked. In the circuitarrangement, a first resistance across which the rectified output voltage of the noise amplifying tube is set up, is connected in series with a source supplying a substantially 2,756,328 Pass??? Jul 1 5 constant direct voltage which counteracts the said rectitied outpiit voltage in the circuit of the grid of each said discharge tube. Theaforementioned voltage source is preferably constitutd by as'econd resistance included between the cathode of' the noise amplifying tube and a point of constant'potential (ground), so that it is traversed by the cathode current of said tube. It is advantageousto rectify the noise voltage amplified in'the noiseamplifying tube by means of a diode circuit including a third resistance connected in series with the first resistance between the point of constant potential (ground) and the grid of each blocking tube in the further receiving channel. In "this case,"a diode-pentode, or a pentode comprising an additionalelectrode which is activeas adiode-anode, 'niay be utilized as the noise amplifying tube.

1 In order to prevent the grid of the blocking discharge tube the further channel from acquiringan unduly high positive voltage, as a result of which grid currents and distortion might occur, provision may be made of a limiter diode which becomes current-conductive'inme case of an unduly high voltage of the said grid 'to counter act 'the increas'ein' grid voltage. I In order that the invention may be readily carried into effect, it will now be described in'greater detail 'with'refer- V ence to the 'accompanying drawing, wherein "thefiguie is stages and a frequency-conversion stage.

a schematic diagram of an embodimentrangement of the present invention. V In the figure; an input circuit 1, which may comprise, for example, one or more high-frequency"amplifying The intermediate-frequency output voltages thereof are supplied to a device2, which comprises a frequency detector in theme of reception" of frequency-modulated oscillations and an amplitude detector in the case or amplitude-modulated oscillations. The" low-frequency output voltages of "the device 2 are supplied by way of a' condenser=3 to the first control grid of a low-frequency a'mplifyingtubet4, which is shown as a pentode. The amplified low-frequencyoscillations are supplied, if necessary; :by way? of further amplifying stages to an output stage. Therdcvice 2 may comprise automatic volume control-known as such. e

S trong noise voltages occur in the output circuit of device 2 in the absence of a signal. In order tosuppress the noise voltage it is supplied by Way of a condensenS to the first control-grid of a noise amplifying tube, 6'. 1 In a manner utilizing the tube 6 described hereinafter, in the absenceof a signal, and hence when the noisevolt'ages are at. a maximum, the first control grid of the low' frequency amplifying'tube 4 has applied to it a negative poof the ai' tential such that the tube 4 becomes inoperative.

Qondenser 5 preferably has a comparatively low capacity of, for example, some tens of micro-microfarads, a resistance 15.of about 0.1 megohm being provided between the control grid and the cathode of the tube6. *The condenser 5 thus substantially passes the frequencies: located above the audible range. It could be replaced by a-high-pass filter, The amplifiednoise voltages are jsupplied by way of a condenser 11 to'a diode-anode 9, which utilizes a common cathode with the pentode-portion of tube 6. Consequently, a direct voltage, which is dependent upon the amplitude of the noise voltages, is set up across the resistance 10 included between the cathode and the anode 9 of the tube 6. Furthermore, the parallel combination of condenser 7 and resistor 8, the latter preferably being variable, is provided between the cathode and ground.

The pentode anode lead includes a resistor 13 and the screen-grid lead includes a resistor 12, decoupled by means of a condenser 14.

The direct voltage with respect to ground set up at The proportioning is such that, in the 5 The difference voltage is supplied by way of a filter constituted by resistances 16 and 17 and condensers 18 and 19, and by way of a grid resistance 20 to the first controlgrid of the low-frequency amplifying tube 4, which is thus blocked. The cathode lead of the tube 4 may include the parallel combination 22 of a resistance and a condenser, in order to provide the correct biasing potential for the control grid.

When a signal voltage occurs in the input circuit of the noise voltage decreases, the voltage at the resistance 8, so far as the action upon the control grid of the tube 4 is concerned, exceeds the voltage at the resistance 10, which in the meantime decreases. Consequently, the biasing potestial of the control grid of tube 4, which at first has a negative value such that the tube is blocked, decreases so that said tube normally becomes operative. If the voltages set up at the resistances are comparatively high with respect to the voltage at the grid of the tube 4, at which said tube is blocked, this change is very abrupt.

If, for example, it is assumed that in the blocked state of the tube.4 the voltage across the resistance 8 is 40 volts and that the alternating noise voltage is 35 volts R. M. S., then, in the absence of a signal, the voltage across the first condenser of the smoothing filter will be The tube 4 is then blocked or substantially inop- If, now, the noise voltage decreases by 20%,

volts. erative.

arrangement of the present invention, due to which the due to the reception of a carrier-wave of the correct frequency, the additional negative grid-bias of the low frequency amplifier tube 4 is 400.8 35 /2%4040=0 volts, so that said low-frequency tubeis unblocked or completely open. The effect may be increased by applying the blocking voltage to the grids of any following lowfrequency amplifying tubes. The optimum effect may be obtained by variation of the resistance 8.

The tube 6 preferably operates as a pure amplifier having a substantially constant grid-bias. In order that the voltage at the resistance 8 may be maintained more accurately at the optimum value, the upper end of said resistance may be connected by way of a resistance 23 t the screen grid of the tube 6.

The circuit-arrangement of the present invention further comprises means for preventing the control-grid of the tube 4 from acquiring a positive bias, in the case of a strong signal, which might lead to the occurrence of grid current and distortion. For this purpose use is made of a diode rectifier 24, the anode of which is connected to the point from which the low-frequency tubes obtain their grid voltages. For this purpose, as an alternative, the tube 4 may be a diode-pentode, the diode electrode fulfilling the said function. In this case it is desirable that the diode anode be biassed at about 1 volt positive withrespect to the control grid since the diode does not become fully conductive until the voltage between anode and cathode exceeds about +1 volt, which, if allowed to be applied to the control grid, might produce undesired effects. An advantage of the circuit-arrangement is that a small number of additional discharge tubes is required.

Itis to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

What I claim is:

1. In a radio receiver including a detector whose output'is fed to an amplifying channel and wherein the noise level at the output of the detector decreases substantially upon occurrence of a signal voltage, apparatus for suppressing noise in said receiver comprising a noise amplifying stage coupled to the output of said detector to produce a noise voltage in the absence of a signal voltage, said stage including an electron discharge tube having a cathode, a grid and an anode, means coupling said grid to the detector output to apply said noise voltage thereto,

:- rectifying means including a first resistance coupled to said anode to produce a direct noise voltage across said first resistance, a second resistance connected between said cathode and ground to produce a substantially constant voltage, means connecting said first and second resistances in series for combining in opposite polarity said constant voltage and said noise voltage to produce a control voltage thereacross, and means to apply said control voltage to said amplifying channel to block same only during the existence of said noise voltage.

2. An arrangement, as set forth in claim 1, wherein I said electron discharge tube further includes a screen grid,

and a third resistance connected between said screen grid and the junction of said series-connected first and second resistances.

3. An arrangement, as set forth in claim 1, wherein said rectifying means includes a diode having a cathode common with the cathode of said tube and a plate incorporated in said tube, said first resistance being connected between said plate and said cathode, said plate being coupled to said anode.

4. An arrangement, as set forth in claim 1, further including a limiter diode coupled across said series-connected resistances to prevent said control voltage from exceeding a predetermined positive value.

References Cited in the file of this patent UNITED STATES PATENTS 2,343,115 Noble Feb. 29, 1944 2,370,216 Worcester Feb. 27, 1945 2,409,139 Magnuski Oct. 8, 1946 2,507,432 Bass May 5, 1950 2,527,617 Berger Oct. 31, 1950 OTHER REFERENCES "Technical Discussion on the Application of the Type Tube to Interchannel Noise Suppression; Laboratory Series No. UL-4; August 25, 1932, RCA Radiotron Co. Manual, 5 pp. spec. 3 shts. Dwg. 

